There are a variety of different head mounted display technologies used for augmented reality and virtual reality applications. A general problem with head mounted display technologies is obtaining a combination of high image quality, compact size, and a reasonable price point. For example, many head mounted displays extend far out in front of a user's face. For example, some head mounted displays extend out ten centimeters or more from a user's face. Many designs suffer from poor brightness or image quality issues. Additionally, cost is an important consideration in consumer applications.
These considerations are also important in a head-mounted projection display (HMPD) in which image projectors attached to a head-mounted frame project light. Many of these approaches also suffer from one or more problems, including the HMPD extending too far out from the user's face, poor brightness, poor image quality, or high cost. It is also difficult in a HMPD to simultaneously obtained a combination of low extension, high brightness, and high image quality.
One type of HMPD utilizes 45 degree beam splitters to direct projected light out to a retroreflective screen and allow reflected light to be transmitted to a user's eye. FIG. 1A shows a prior art arrangement similar to U.S. Pat. No. 5,606,458. This type of HMPD utilizes a pair of 45 degree beam splitters to direct unpolarized light. In this type of HMPD, the HMPD has one beam splitter placed in front of each of the user's eyes. Each image projector directs projected light through a respective beam splitter and out to a retroreflective screen. The retroreflected light reflects back to the same beam splitter it originated from and the beam splitter directs the reflected light into the user's eyes. FIG. 1B shows in more details aspects of the 45 degree beam splitter. A projector 101, a beam splitter 102 and a retroreflective screen 103 are used to present images to the eye 104 of an observer.
However, the approach of FIG. 1 results in significant loss of brightness, and therefore image contrast, at each pass on/through the beam splitter. The first 3 dB loss path 105 passes through the beam splitter having come from the projector 101. The second 3 dB loss path 106 directs light back into the projector after retroreflection resulting in a combined 6 dB total loss.
In order to reduce these losses, U.S. Pat. No. 5,621,572 and U.S. Pat. No. 8,259,239 developed an improved arrangement similar to that shown in FIG. 2A in which a polarizing beam splitter is used with an additional waveplate. FIG. 2B shows the arrangement of the 45 degree beam splitter and the waveplate in more detail. The projector 201 produces images having plane polarized light (indicated by the “P” designation on the drawing) that reflects at low loss off the front surface of a polarizing beam splitter 202, with a matching polarization orientation, and then passes through a quarter waveplate 203 with fast and slow axes set at 45 degrees from the incident light plane polarization axis, thus converting the polarization from a plane polarization to circular polarization before rebounding from retroreflecting screen 103.
The return path of FIG. 2B takes the light back through 203 where it is converted back to plane polarization, however, having passed twice through the quarter waveplate, the plane of polarization is now rotated ninety degrees, which then passes through the polarizing beam splitter 202 and on to the user's eye 104 with low loss. Those of skill in the art will notice that the reversal of the circularly polarized light with change of handedness results in an added 180 degree rotation of the plane of polarization when the light passes back through the quarter waveplate, but this added flip is no restriction to the continued propagation through 202.
However, a disadvantage of the positioning of the waveplate is that it causes an unwanted extension of the front of the head mounted unit. This can be seen in FIG. 2A and is represented by distance 204 in FIG. 2B. This includes the thickness associated with the waveplate, optical mounts, spacers, vibration dampeners, or other fixtures. In a commercial product the optics have to be comparatively rugged and cheap to manufacture and thus there are practical limitations on how closely optical components can be placed. Additionally the spacing between components has to take into account the need for light from an image projector to diffract, reflect off the beam splitter, and strike the waveplate. The combination of all of the factors leads to a considerable increase in the forward extension. It is believed that this increases the forward extension in the range of 1 cm to 3 cm over a basic design having a 45 degree beam splitter but no waveplate. As a result, the total forward extension is unacceptable for many consumer applications. For example, FIG. 5B of U.S. Pat. No. 8,259,239 shows a thickness of 5 centimeters in a region above a user's brow.
Additionally, the approach of FIG. 2A and FIG. 2B requires a manufacturing step to mount and align the wave plate and associated fixtures. This increases the cost of the HMPD.
Moreover, the approach of FIGS. 2A and 2B introduces the potential for unwanted reflections off the inside surface of wave plate 203. Also, not all rays in the projected Field of View (FoV) will strike the waveplate at an orthogonal angle of incidence which can result in imperfect phase retardation and chromatic distortions. These effects also reduce the image quality of the images that make it to the user's eyes.